So, with regards to this commonly stated belief of "the sittinv president never endorses before his own party's primary is over. It's not kosher. Obama will wait for Sanders to drop." I didn't really buy it, so I looked it up. First of all, in the past 30 years, there's only been three opportunities for it to happen anyway, so not enough to really claim a strong trend. But more importantly, the president has endorsed before the process was over in all three cases. Hell, Clinton endorsed Gore in December, before any votes had been cast. If Obama actually waited for Sanders to drop, it would be the exception, not the norm.
In addition, Reagan endorsed H.W. while Robertson was still in, and Dubya endorsed McCain while Romney, Huckabee, and Paul were still in.
Point being, I expect Obama to endorse Clinton after next Tuesday, but it wouldn't have been weird if he had done it before.